<!DOCTYPE html>
<style>
#outerDiv {
  width: 500px;
  height: 500px;
  background: lime;
  position: absolute;
}

#middleBlur {
  width: 300px;
  height: 300px;
  margin-left: 100px;
  margin-top: 100px;
  background: black;
  box-shadow: inset 0 0 20px 100px lime;
}
</style>

<div id="outerDiv">
  <div id="middleBlur">
  </div>
</div>
